1. Field of the Invention
The present invention relates to a liquid droplet ejecting device which ejects liquid droplets, and in particular, to a liquid droplet ejecting device which is optimal for forming images by ejecting ink droplets at an FWA-type inkjet recording head.
2. Description of the Related Art
The development of inkjet recording devices, which use an FWA (Full Width Array) type inkjet recording head (an FWA head) equipped with ejecting nozzles which are lined-up in an axial direction orthogonal to the conveying direction of the recording medium, has evolved more and more in recent years in order to record at high speeds. In such an inkjet recording device, forming the arrangement of the ejecting nozzles two-dimensionally is effective in order to obtain a high resolution.
In such an FWA head which is formed two-dimensionally, there are many cases in which there are portions (discontinuous portions) where the distance, in a sheet conveying direction, between ejecting nozzles which are adjacent to one another in the axial direction, is large (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-145777 and 2003-170645).
Further, the sheet must be conveyed at a right angle with respect to the FWA head. If the sheet is not conveyed at a right angle, on the sheet, the aforementioned interval in the axial direction of the discontinuous portion differs from other portions, and when the interval widens, a white stripe forms, and when the interval narrows, a black stripe forms, and the image quality deteriorates. Therefore, the FWA head and the sheet conveying unit must be positioned with accurate perpendicularity therebetween. However, what is important is the perpendicularity between the FWA head and the sheet conveying direction (belt conveying direction). If this perpendicularity is not met, it will be insufficient even if the perpendicularity of the FWA head with respect to the sheet conveying unit is achieved. Detailed explanation will be given hereinafter by using examples and by referring to the drawings.
As shown in FIGS. 30A, 30B, 31A, 31B, 32A, and 32B, an inkjet recording head 103 is an FWA-type inkjet recording head which is elongated and at which the arrangement of ejecting nozzles 98 is formed two-dimensionally.
The ejecting nozzles 98 form plural ejecting nozzle rows 99. The ejecting nozzle rows 99 are lined-up along a direction V so as to be parallel to one another.
In intervals between adjacent ejecting nozzles 98, components thereof in an X direction, which is orthogonal to a conveying direction Y of a sheet P, are uniform. As shown in FIGS. 30A and 30B, when the inkjet recording head 103 is orthogonal to the conveying direction Y of the sheet P (i.e., when the longitudinal direction of the inkjet recording head 103 is parallel to the X direction), the aforementioned component in the X direction (the X direction nozzle pitch) is ΔX.
In intervals between adjacent ejecting nozzles 98, components thereof in the Y direction, which is parallel to the conveying direction Y of the sheet P, are uniform in the same ejecting nozzle row, but are greater in the Y direction in each of adjacent ejecting nozzle rows. As shown in FIGS. 30A and 30B, when the inkjet recording head 103 is orthogonal to Y direction, the aforementioned component in the Y direction (the Y direction nozzle pitch) is AY, and the aforementioned component in the Y direction in each of the adjacent ejecting nozzle rows is greater, that is, a distance LY.
As shown in FIGS. 31A and 31B, in a case in which the inkjet recording head 103 is, as compared with the state shown in FIGS. 30A and 30B, tilted by being rotated left in the drawing (paper) by angle θ and the X direction width of the ejecting nozzle row 99 narrows and becomes W-ΔW, the X direction width of adjacent ejecting nozzles 98 is ΔX-e1 in the same ejecting nozzle row, and is ΔX+e2 at the adjacent ejecting nozzle rows.
As shown in FIGS. 32A and 32B, in a case in which the inkjet recording head 103 is, as compared with the state shown in FIGS. 30A and 30B, tilted by being rotated right in the drawing by angle θ and the X direction width of the ejecting nozzle row 99 widens and becomes W+ΔW, the X direction width of adjacent ejecting nozzles 98 is ΔX+e1 in the same ejecting nozzle row, and is ΔX-e2 at the adjacent ejecting nozzle rows.
Given that the width of the inkjet recording head 103 in the Y direction (the sheet conveying direction) is B, the following formulas are established.e1=ΔX(1−cos θ)−ΔY sinθe2=ΔX(cos θ−1)−B sin θpitch error=e1+e2=(B−ΔY)sin θ
Here, given that B=30 mm and ΔY=0.5 mm, the pitch error (e1+e2) of ΔX, at θ=0.00033 rad (0.1 mm/300 mm), is 10 μm.
When the pitch error becomes as large as about 10 μm, white stripes 104 (see FIGS. 31A and 31B) or black stripes 106 (see FIGS. 32A and 32B) are conspicuous and problematic.
In particular, in a device which obtains full-color images, because the FWA head is structured by plural ejecting nozzles being lined-up, a unit which holds a recording sheet to a conveying belt and conveys the recording sheet is used as the aforementioned sheet conveying unit. Generally, a conveying belt is provided with a walking preventing section so that the belt does not move in the direction orthogonal to the conveying direction. In the present specification, the belt moving in the direction orthogonal to the conveying direction is called walking. There is a method in which guide members are adhered to the end portions of the belt such that walking of the belt is prevented at the guide members (and also a method of directly guiding the belt end portions by the guide members), and a method in which walking of the belt is prevented by tilting a driven roller (a steering method). In the case of preventing walking by providing guide members, because the accuracy of the guide members affects the belt walking, usually, conveying can be carried out at a walking preventing accuracy of about 0.1 to 0.2 mm. On the other hand, in the steering method, the end portion of the belt is detected at a sensor, and the tilting of the roller can be controlled electrically. Therefore, fine control is possible, and walking can be controlled at an accuracy of less than or equal to 0.05 mm. In a device for full-color images, in order to prevent miss color registration (miss color registration must be kept to less than or equal to 0.1 mm), belt walking must be controlled highly accurately, and the steering method is effective. However, even when such a belt walking preventing section is provided and walking of the belt is prevented, the belt conveying direction is not perpendicular to the axis of the roller. Namely, there is a case in which the belt conveying direction is not perfectly perpendicular to the axis of the roller but is slightly different from a direction which is perpendicular to the axis of the roller. In the present specification, this is defined as belt skewing. The belt conveying direction is problematic not only in belt skewing (see FIG. 8), but also in cases in which the belt conveying device is at an angle with respect to the FWA head (see FIG. 10).
Accordingly, the belt conveying direction (angle) is important.
The belt conveying direction changes due to various causes such as the state of the conveying belt, the environment, the setting conditions, and the like. Therefore, there are cases in which the belt conveying direction changes at the time of assembly at the factory, at the time of set-up, at the time of start-up, at the time of continuous printing, and the like. Thus, it is important to always maintain the belt conveying direction at a predetermined direction (in many cases, orthogonal) with respect to the FWA head, and to prevent walking and skewing from arising at the conveying belt.
This is not limited to FWA-type inkjet recording heads, and the same holds for inkjet recording devices equipped with PWA-type inkjet recording heads. Moreover, this is not limited to inkjet recording devices, and the same holds when conveying an object-of-adhesion, to which ejected liquid droplets are to be adhered, by a conveying belt at a ejecting side of a liquid droplet ejecting head even in a liquid droplet ejecting device which has a liquid droplet ejecting head in which ejecting nozzles for liquid droplet ejecting are arranged two-dimensionally.